A fourth person has died from injuries suffered when a boiler exploded at a St. Louis business and crashed through the roof of a nearby business, the St. Louis medical examiner's office said Thursday.

Clifford Lee, 53, of St. Louis, died Wednesday at St. Louis University Hospital, the medical examiner said.

Lee was with two friends Monday filling out new employee paperwork at Faultless Linen Co. when the one-ton boiler exploded at nearby Loy-Lange Box Co. flew through the air and crashed through the linen company's roof, The St. Louis Post-Dispatch reported.

His friends, Christopher Watkins, 46, and Tonya Suarez-Gonzalez, 43, a married couple from St. Ann, Missouri, died at the scene.

A Loy-Lange engineer, Kenneth Trentham, 59, also died in the blast. Trentham was one of three engineers at the company licensed to operate boilers. He was hired to work at Loy-Lange in 1996, according to Maggie Crane, a spokeswoman for St. Louis Mayor Francis Slay said.